In this emerging field, researches are performed to closely integrate all the basic elements (laser sources, waveguides, modulators, photodetectors) necessary to create photonic functions on top of a CMOS silicon wafer (Complementary Metal Oxide Semiconductor silicon wafer). One of the major goal of these Photonic Integrated Circuits (PIC) is to increase the bandwidth of IC systems, which is up to now limited by electrical interconnects.
From a material point of view, it is well known that silicon (into SiO2) can be used efficiently to create waveguides in a CMOS layer, to route the light in and out of the circuit, with a telecom wavelength comprised between 1.3 and 1.55 μm. However, silicon has a very poor photon emission efficiency.
Therefore, in order to realize laser sources and photodetectors on silicon, a promising and compact approach is to integrate III-V materials, known to have very good photon emission and detection efficiencies, on top of the CMOS layers, by means of die to wafer bonding on silicon wafer. A reference can be found in G. Roelkens et al., III-V/Si photonics by die-to-wafer bonding, Materials Todays 2007, vol.10, n° 7-8, pp. 36-43.
On one die, whose typical dimension is 5×5 mm2, thousands of devices, for instance lasers, can be defined.
A typical structure includes a CMOS silicon wafer on the top of which is bonded a die with an active material which, after processing, will define at least one laser and one photodetector. The CMOS silicon wafer comprises a stack of a silicon substrate, an oxide buffer layer and a silicon waveguide layer. In the literature, the laser and the photodetector are both made of the same III-V epitaxial layer stack. Therefore, the laser and the photodetector include the same layers. A reference can be found in G. Roelkens et al., laser emission and photodetection in a InP/InGaAsP layer integrated on and coupled to a Silicon-on-Insulator waveguide circuit, Optics Express 2006, vol. 14, n° 18, pp. 8154-8159 or A. Fang et al., Integrated AlGaInAs-silicon evanescent racetrack laser and photodetector, Optics Express 2007, vol. 15, n° 15, pp. 2315-2322.
By using evanescent coupling, it is possible to couple the light emitted by the laser in the underneath Si/SiO2 waveguide, and further detect it in a photodetector (with the same active layer) using the same principle, therefore realizing a full optical link on silicon.
As it is known in the state of art, evanescent wave coupling is the process by which electromagnetic waves are transmitted from one medium to another by means of the evanescent, exponentially decaying electromagnetic field. In the present case, the laser generates an electromagnetic field which extends to the underlying waveguide. In other words, the spatial distribution of the electromagnetic field is such that part of it reaches the waveguide. If the laser and the waveguide are closely located, the evanescent field generated by the laser does not decay much before it reaches the waveguide.
Concerning evanescent coupling, reference is made to H. Park et al., Hybrid silicon evanescent laser fabricated with a silicon waveguide and III-V offset quantum wells, Optics Express-2005, vol. 13, n° 23, pp 9460-9464 and to H. Hattori et al., Heterogeneous integration of microdisk lasers on silicon strip waveguides for optical interconnects, IEEE Photonics Technology Letters 2006, vol. 18, n° 1, pp. 223-225. G. Roelkens et al., Laser emission and photodetection in a InP/InGaAsP layer integrated on and coupled to a Silicon-on-Insulator waveguide circuit, Optics Express 2006, vol. 14, n° 18, pp. 8154-8159 and A. Fang et al., Integrated AlGaInAs-silicon evanescent racetrack laser and photodetector, Optics Express 2007, vol. 15, n° 5, pp. 2315-2322, are also of interest.
The advantage of this configuration is that a unique bonding step is necessary to realize both lasers and photodetectors with exactly the same III-V layers.
However, the drawback of this configuration is that the lasers and photodetectors performances cannot be optimized independently, in terms of layer thickness and composition. Indeed, the same p-i-n structure is used in one area of the die for emission (p-i-n forward biased) and in another area of the die for detection (p-i-n reverse biased).
This configuration is illustrated by Roelkens G et al., Thin film III-V devices integrated on silicon-on-insulator waveguide circuits, ECS transactions, Science and technology of dielectrics for active and passive photonic devices 2006 Electrochemical Society Inc. US, vol. 3, no. 11, 2006, pages 101-106, and the article of A. Fang et al, mentioned above.
By using the same die-to-wafer technique, it is possible to dedicate dice to laser emission and other ones to photodetection. Reference is made in this regard to work achieved in the frame of the European project PICMOS. The dice, whose composition is different, are optimized for each function, either emission or photodetection.
Therefore, it requires two bonding steps: one for laser dice and one for photodetector dice.
This technique is illustrated by Van Thourhout D et al., III-V silicon heterogeneous integration for integrated transmitters and receivers, Proceedings of Spie, The International Society for Optical Engineering, Integrated Optics: devices, materials, and technologies XII 2008 Spie US, vol. 6896, 2008.
However, the major drawback of this technique is that the distance between a source and a photodetector is ruled by the size of the dice (usually 5 mm×5 mm) and the minimum distance allowed to bond dice (usually several millimetres). Therefore, this technique cannot be used for some important and common applications where, for instance, a monitoring photodiode has to be implemented in close proximity of a laser, that is to say, at a distance of tens of μm.
It is therefore an object of the invention to obviate these drawbacks by proposing a method of producing a photonic device including at least one light source and at least one photodetector on a silicon wafer, this method including a unique bonding step, while enabling the optimization of the performances of the light source and of the photodetector and a close positioning of the light source and of the photodetector.